Glossary
A/B Update
A servicing type where the host follows the A/B partition scheme: volumes are present as identical copies, A and B, and while the active partition is running workloads, the OS image on the inactive one is updated. The host will then reboot into the updated partition, while the other one becomes inactive.
To be eligible for A/B updates, a volume must be present as two identical block device copies on the disk: A and B. These device copies form a logical A/B Volume Pair. Other volumes might be present as a single copy shared between the A and B partitions, but they are then ineligible for A/B updates.
A/B Volume Pair
A pair of block devices that are used for an A/B update. One volume is the A volume, and the other is the B volume. At any point in time, only one volume is active, and the other is inactive.
An A/B Update is performed by updating the inactive volume, and then rebooting the device into the updated volume. When this happens, the active volume swaps from A to B, or from B to A.
A system can have multiple A/B volume pairs, each pair representing a different mount point on the device. All pairs in an install are updated in lockstep, meaning all pairs will have their A volume be the active one, or all pairs will have their B volume be the active one.
Block Device
Kernel abstraction generally used for non-volatile storage devices, such as hard drives, SSDs, and USB drives.
A file that refers to a device. A block special file is normally distinguished from a character special file by providing access to the device in a manner such that the hardware characteristics of the device are not visible.
Clean Install
A servicing type where a new install is performed.
A clean install does not update or modify an existing OS. It creates an entirely new install on the device.
A clean install is what you do when you install an OS for the first time, or when you are replacing an existing OS with a new one.
Dualboot
See Multiboot.
Execroot
Execution root. The root file system of the environment where Trident was run. Generally the Management OS, the OS that is being updated, or a container running on top of one of the former environments.
Finalize (Operation)
The finalize operation performs any final pre-reboot actions needed for the servicing, as well as the reboot itself.
Install
A full deployment of a Azure Linux made with Trident.
The install encompasses the entire OS, including the bootloader, the kernel, the initramfs, the root filesystem, all A/B Volume Pairs, associated partitions, and any other partitions that are part of the install.
Note: This definition does not consider other OSes or distros.
minimal-os
minimal-os is an Azure Linux vhdx maintained by Microsoft, comprised of the minimal components to be bootable.
Multiboot
The capability of having multiple installs on the same device, even on the same disk.
Newroot
Root file system of the OS that is being deployed.
When Trident is deploying a target OS, it will mount the target OS's file
systems and prepare them for a chroot. This mount of the target OS is called
newroot.
Operation
Operations are the top level actions performed during servicing. Trident installations and updates perform the stage and finalize operations.
Servicing
The general process of performing an action on an install. There are several types of servicing.
Servicing OS
The OS where Trident is running.
Servicing Type
The specific kind of Servicing that is being performed on an install, such as clean install, or an A/B update.
Stage (Operation)
Stage is an operation that downloads, writes and prepares an OS image as part of a servicing.
Step
Steps are logical phases of an operation. On each step, the method of each subsystem relevant to the step is run to perform the work needed for that step.
Subsystem
A logical grouping of related functionality within Trident. Each subsystem is in charge of a specific aspect of the servicing process and configuration of the newroot. Subsystems run the corresponding logic for each step of an operation. Trident contains several subsystems handling different aspects of the servicing process, such as storage configuration, OS configuration, network configuration.
Target OS
The OS being serviced to reach the desired state, defined in the Host Configuration.
On a clean install, this is the new OS being installed to disk. On A/B update, this is the new partition being provisioned. For runtime configurations, the servicing OS and the target OS are the same.
Unformatted Partition
An unformatted partition is a partition on a storage device that has been created but does not yet contain a filesystem. It is not associated with any filesystem, verity-filesystem, RAID array, or encryption volume.